RAID 50 with 18 disks — 16 usable

RAID 50

Several RAID 5 groups striped together. Each group spends one disk on parity, so more groups means less capacity and more failures survived.

TB on the label, TiB on the screen

• 1 TB × 1816 TB · 14.55 TiB
• 2 TB × 1832 TB · 29.1 TiB
• 4 TB × 1864 TB · 58.21 TiB
• 6 TB × 1896 TB · 87.31 TiB
• 8 TB × 18128 TB · 116.42 TiB
• 10 TB × 18160 TB · 145.52 TiB
• 12 TB × 18192 TB · 174.62 TiB
• 14 TB × 18224 TB · 203.73 TiB
• 16 TB × 18256 TB · 232.83 TiB
• 18 TB × 18288 TB · 261.93 TiB
• 20 TB × 18320 TB · 291.04 TiB
• 22 TB × 18352 TB · 320.14 TiB
• 24 TB × 18384 TB · 349.25 TiB

One TB on the box is 10¹² bytes, while the operating system counts in units of 2⁴⁰ bytes. The names look alike but the units differ, so the same disk reads about 9% smaller. Nothing went missing — the ruler changed.

How the groups are split

• Groups 2 · Disks per group 916disks · 2
• Groups 3 · Disks per group 615disks · 3
• Groups 6 · Disks per group 312disks · 6

For RAID 50 and 60, how many groups you use is the real decision. Fewer groups spend less on parity and leave more capacity; more groups leave one (or two) spare disks per group, so more failures are survivable when they land well. The guaranteed figure does not change either way — two failures inside one group end it regardless.

Same disk count, other levels

Same level, other disk counts

The rebuild is the dangerous part

When a single-parity array loses a disk, you put a new one in and every remaining disk is read end to end to rebuild it. That is precisely when the survivors work hardest and longest. The bigger and more numerous the disks, the longer the rebuild — days, sometimes — and a second failure during it ends the array. This is why RAID 6 or 10 is preferred over RAID 5 on large arrays.

How to read it

• Usable = total − parity × groups. Every level is that one line.
• RAID 5 spends one parity disk in a single group, so n−1; RAID 6 spends two, so n−2.
• RAID 50 and 60 run several such groups striped together, so the parity is multiplied by the group count.
• RAID 1 holds one copy on every disk, so one disk’s worth; RAID 10 mirrors in pairs, so half.
• Groups must be equal in size, so the group count can only be a divisor of the disk count.
• TB on the box and TiB on the screen are different units — about 9% apart.

RAID is not a backup

RAID protects against exactly one thing: a disk dying. A file deleted by mistake, a file overwritten, ransomware, a failed controller, fire or theft all propagate to every disk at once — a mirror deletes just as faithfully as it writes. A backup is a separate copy on separate hardware, and RAID is not doing that job.